1. Field of the Invention
The present invention relates to a wide-angle optical system for a solid image pickup device. In particular, the present invention relates to a wide-angle optical system for a solid image pickup device suitable for use in an image-taking device that uses a solid image pickup device mounted on a portable phone, a PC camera, and the like.
2. Description of the Related Art
In recent years, with the development of mass-media, there has been an increasing demand for cameras that utilize a solid image pickup device, such as a CCD, CMOS, or the like, which are mounted on a portable computer, a television phone, or a portable phone, for example.
It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the optical system used for such cameras to be small and light, as is the camera.
When only aiming for higher performance, an optical system using two lenses or three lenses is may be used as the optical system used in such cameras. However, in order to actualize size reduction and cost reduction, a single lens optical system is often used. An optical system including imaging lenses such as a biconvex lens, a meniscus lens, or the like is used as the single lens optical system (for example, refer to Patent Literatures 1-6).    [Patent Literature 1] Japanese Patent Unexamined Publication Heisei 6-88939    [Patent Literature 2] Japanese Patent Unexamined Publication Heisei 10-282410    [Patent Literature 3] Japanese Patent Unexamined Publication Heisei 11-264930    [Patent Literature 4] Japanese Patent Unexamined Publication Showa 61-116314    [Patent Literature 5] Japanese Patent Unexamined Publication Heisei 6-118300    [Patent Literature 6] Japanese Patent Unexamined Publication Showa 64-49014;
As an aberration that conventionally occurs in an imaging optical system, an aberration that warps an image (particularly an image that is off-axis), referred to as distortion, is known.
The distortion is an aberration that is proportional to the third power of the view angle. Therefore, the distortion is not a significant problem in an optical system with a narrow view angle. However, the distortion is an extremely significant problem in a wide-angle optical system.
For example, the single lens optical systems described in Patent Literatures 1 to 3 have a view angle (view angle of corresponding angles) of about 60°, which is a sufficiently wide view angle for a wide-angle optical system used in a solid image pickup device. The single lens optical systems also have superior resolutions. However, a large distortion occurs because of the wideness of the view angle.
The single lens optical systems described in Patent Literature 4 and 5 are optical systems proposed for use in various sensors or photometry. Therefore, the intended use of these single lens optical systems greatly differs from the optical system for the solid image pickup device.
The single lens optical system described in Patent Literature 6 is for narrow view angles (for example, the maximum view angle is about 35°) and is not suitable for wide-angle optical systems at all.
In this way, conventionally, the distortion could not be sufficiently corrected in the wide-angle optical system for solid image pickup devices.
Therefore, the present invention has been designed in view of the aforementioned problems. The object of the present invention is to provide a wide-angle optical system for a solid image pickup device that can effectively correct distortion.